Data storage in general, but most advantageously in Magneto-Optical data storage utilizing linear data storage without significant SNR loss.
It is well known in communications engineering that a linear channel has considerably more capacity for information transmission than a saturation channel. Data storage channels are invariably saturation channels. Efficient linearization of these channels enables a considerably larger storage capacity and lower cost per MegaByte of stored data. A primary feature of the above-identified application is that MO media has an amorphous film nature and is vertically oriented; the two together are attractive for linearized channel data storage SNR degradation is minimized.
The issue is how to avoid taking an amplitude reduction over what would have occurred with saturation recording. In other words, given the analog signal, which can take on a continuous range of values between its two peak extremes (max and min) one should place that max and min right inside or very close to the max and min extremes utilized in binary saturation recording, and keep the same dynamic range available for the continuous time signal, thereby not suffering peak signal noise.
In the past, linearization of saturation magnetic recording channels was done through the use of AC bias techniques. In the prior art, the saturation channel was driven by a transmitter or xe2x80x9cwritexe2x80x9d driver that formed the sum of the analog signal to be recorded and a sinusoidal high frequency bias signal. The frequency of the sinusoidal bias signal was generally greater than three (3) times that of the highest frequency component in the analog data to be recorded. In the receiver or xe2x80x9creadxe2x80x9d channel the bias frequency is rejected by the low-pass response of the channel and only the analog signal is received. The problem with the prior art of linearization of saturation channels is that there is typically 6 to 7 dB of SNR (signal-to-noise ratio) loss in the analog signal processing from that achievable in two-level saturation signaling. If one attempts to increase the analog signal level to improve the SNR, non-linearity causes excessive generation of inter-modulation products. Moreover, noise is further increased in modem thin metallic film media by the higher flux transition densities caused by the presence of the high-frequency AC bias in the xe2x80x9cwritexe2x80x9d process. This large SNR loss due to these factors is difficult to overcome with the more efficient linear channel signal processing, and has inhibited the application of advanced signal processing techniques to data storage.
Efficiency of linearization of saturation channels is improved so that there is only 1 to 2 dB of SNR loss in practical implementations of the invention. The process of conventional AC bias linearization is most simply viewed as Duty-Cycle Modulation (DCM) of a high frequency signal in the transmitter. The receiver acts as limiter with hysteresis that is small relative to the amplitude of the duty-cycle modulated high frequency AC bias waveform. This saturation non-linearity is followed by a low-pass filter. Duty cycle modulation occurs along the linear portion of the sinusoid as the analog base-band signal is added to the sinusoidal AC bias. The duty-cycle modulation is linear as long as the additive baseband signal is small relative to the sinusoidal AC bias component as the zero-crossing of the total composite waveform is linearly proportional to the amplitude of the analog base-band signal. This produces a duty-cycle modulated waveform linear over the range of about 25% to 75% of a cycle; consequently, this results in a 50% (6 dB) loss of received signal amplitude over that possible with simple saturation signaling. The novel idea presented here results first from the recognition that AC bias linearization is most simply modeled as a duty-cycle modulation. Subsequently, duty-cycle modulator (DCM) is designed to produce a full range of 0% to 100% duty-cycle modulation.
Since the output and input are nearly equal in the base band level, the present invention utilizes the fact that the conventional AC bias was in fact doing pulsewidth modulation, or duty cycle modulation, and in readback the readout was low pass filtering the duty cycle modulated recorded signal and recovering the base band signal. Preferably, according to this invention, a more linear duty cycle modulator is made by converting the sine wave bias into a triangle wave bias so that the sine wave slopes are linear all the way up to the peaks, they turn the corner and then remain linear back to the subsequent peak. As the base band signal is added precisely having its amplitude equal to the amplitude of the triangle wave bias, then the composite sum just barely touches zero coming from above and just barely touches zero coming from below to give you nearly zero width pulses coming from above and below on the limiter; when you low pass filter out the high frequency content of the duty cycle modulated waveform, the base band signal is recovered at full amplitude as if saturation recording was being utilized.
Secondly, magnetic media is used that does not produce added noise as the flux-transition density is increased. The amorphous medium and vertical magnetic recording associated with magneto-optical recording aids the linear magnetic recording process. A small spot on the vertically oriented medium is heated by a focused optical beam to reduce the magnetic coercivity over the region on which information is to be recorded. This is the erasable MFM (magnetic field modulation) magneto-optical recording process. It is ideally suited to the application of linearization through the use of duty-cycle modulation as described here. Because the medium is amorphous, it does not exhibit a prohibitive increase of noise as the flux transition density is increased as is the case with conventional digital magnetic recording on thin metallic films used in current state-of-the-art in disc drives.
A method of reducing the peak signal-to-noise (SNR) requirement in linearized magnetic recording further comprises the use of a compandor coupled with the duty-cycle modulator (DCM). This removes most of the penalty associated with the high peak-to-average power ratios of the bandwidth efficient modulation schemes. Transition noise in the media still remains the chief limiting factor in performance.